TRANSITION-STATE STRUCTURES AND ENERGETICS USING GAUSSIAN-2 THEORY

The availability of the easily implemented Gaussian-2 (G2) methodology has made it possible for the nonspecialist to calculate accurate heats of formation for many molecules on workstations. In order to quantify its performance for transition state structures, we have used G2 and a modified G2 on several transition states whose structures and energies have been well characterized either by experiment or multireference configuration interaction studies. The G2 method performs well in predicting energies of transition states (even for nonisogyric reactions), with an absolute average deviation of 1.5 kcal/mole in the classical barrier height for the cases studied, while it is less successful in predicting geometries and frequencies. We investigated modifying the G2 method for use with transition states by using QCISD/6-311 G (dp) geometries and frequencies instead of MP2/6-31G (d) geometries and scaled HF/6-31G(d) frequencies. The QCISD geometries and frequencies agree well with values from the literature, and this modified G2 procedure offers improved performance in predicting transition state energies.